1. Field of Invention
The present invention relates to a highly reliable two-terminal nonlinear device used as a switching device where the sharpness of the voltage-current characteristic is sufficiently large and change of the voltage-current characteristic is small, a method for manufacturing the two-terminal nonlinear device, and a liquid-crystal display panel having high image quality without display irregularity or sticking and with high contrast that uses the two-terminal nonlinear device.
2. Description of Related
In an active matrix-type liquid-crystal display device, liquid crystal is filled between an active-matrix substrate which forms a matrix array by disposing a switching device for each pixel area and an opposing substrate in which a color filter is disposed, for example. Alignment of the liquid crystal for each pixel area is controlled to display desired image information. In general, a three-terminal device such as a thin film transistor (TFT) or a two-terminal device such as metal/insulating material/metal (MIM) nonlinear devices is used as a switching device. A two-terminal device used as a switching device is more advantageous than a three-terminal device since two-terminal devices are free of cross-over shorting and can be manufactured by a simpler process.
Japanese Patent Laid-Open Publication No. Sho 63-50081 discloses a technique for improving nonlinearity of a MIM nonlinear device. In this technique, a tantalum film is subjected to anodic oxidation and heat-treated at a temperature of 400-600xc2x0 C., to improve the nonlinearity, particularly, the sharpness of the voltage-current characteristic. However, sufficient nonlinearity has not been obtained even by this technique.
Therefore, an objective of the present invention is to provide a two-terminal nonlinear device with high reliability having a sharp voltage-current characteristic and a small change of the voltage-current characteristic, and a liquid crystal display panel that uses the two-terminal type non-linear device with high image quality without image irregularity and sticking, and with high contrast.
Furthermore, another objective of the present invention is to provide a method for manufacturing a two-terminal nonlinear device having the above-desirable characteristics.
The two-terminal nonlinear device (hereafter referred to as xe2x80x9cMIN nonlinear devicexe2x80x9d) according to the present invention has characteristics such that water is included in an insulating film in a two-terminal nonlinear device that includes a first conductive film, the insulating film, and a second conductive film which are laminated on a substrate. In the insulating film, in the thermal desorption spectrum, the peak derived from the water of the insulating film is in a range of 225-300xc2x0 C.
Furthermore, the MIM nonlinear device according to the present invention is not limited to a metal second conductive film, and may include an ITO (Indium Tin Oxide) conductive film.
By including water in the insulating film of the MIM nonlinear device according to the present invention, the nonlinearity coefficient (xcex2 value) that represents the sharpness of the voltage-current characteristic is significantly improved.
In the present invention, with respect to the thermal desorption spectrum, the number of molecules which is calculated from the area of the peak derived from the water of the insulating film is preferably 5xc3x971014/cm2 or greater, and more preferably is 1.0xc3x971015-5.0xc3x971015/cm2. Furthermore, the number of water molecules contained in the insulating film shows the average value in the film thickness direction of the insulating film. In addition, by a secondary-ion mass-spectrometry (SIMS) elemental analysis with the irradiation of cesium primary ions, the intensity of the hydrogen spectrum of water preferably changes one peak or more near the insulating film. Furthermore, in the present invention, the hydrogen spectrum of water in the insulating film obtained by SIMS exhibits at least one peak preferably in the vicinity of the surface of the second conductive film of the insulating film, more preferably, within a range from the surface to 30 nm deep.
The first conductive film is preferably tantalum or a tantalum alloy. Further, the insulating film is preferably formed by anodic oxidation of the first conductive film.
The manufacturing method of the MIM nonlinear device according to the present invention includes
(a) a step of forming a first conductive film on a substrate,
(b) a step of forming an insulating film on a surface of the first conductive film by anodic oxidation of the first conductive film using a electrolyte of ethylene glycol including water at a ratio of 1-10% by weight as a solvent,
(c) a step of including water in at least the insulating film by performing a first heat treatment of the substrate on which the first conductive film and the insulating film are formed in an atmosphere including water vapor, and
(d) a step of forming a second conductive film on the insulating film.
According to this manufacturing method, a MIM nonlinear device according to the present invention as described above can be obtained by employing a simple heat treatment. Further, in this manufacturing method, water may be incorporated into the insulating film by the first heat treatment step (c). In addition, in the step (b) of forming the insulating film using ethylene glycol including a specified amount of water as a solution, it is possible to reliably incorporate water into the insulating film.
In the first heat treatment step; the concentration or density of water vapor relative to the entire treatment gas is preferably 0.001.mol% or more, and more preferably 0.014-2 mol%. Preferably, this first heat treatment (annealing step A) is performed as a temperature-descending step continuously after a second heat treatment (annealing step B) in which the substrate provided with the first conductive film and the insulating film is heat-treated in an inert gas. Also, it is preferable that the second heat treatment is performed within a range of 320-380xc2x0 C.
It is considered that the insulating film of the MIM nonlinear device has a structure of joined insulating materials having different energy levels of their conductive segments. As a result, when a low voltage is applied to the MIM nonlinear device, the resistance of the device is large and the xcex2 value is also large. This will be further discussed in detail as follows.
In a MIM nonlinear device manufactured by a method including the above described annealing A, the insulating film has a structure including a first surface layer that contains water and is closer to the film surface facing the second conductive film, and a second layer that does not substantially contain water. Thus, the insulating film has the conductive segments at different energy levels. That is, the conductive segment of the first layer containing water has an energy level lower than that of the second layer without water. Therefore, when a low voltage (for example, 5V or less) is applied to the MIM nonlinear device, the resistance of the device becomes large to dissolve the energy difference between the conductive segments in the insulating film. Meanwhile, when a high voltage (for example, 10V or more) is applied to the MIM nonlinear device, resistance of the device varies little since the energy difference within the insulating film does not substantially influence the electrical conduction. Because of this, the voltage-current characteristic of the MIM nonlinear device is sharpened. At this time, the resistance value R of the device can be expressed by the following formula.
R=1/xcex1exp(xcex2Vixc2xdxe2x88x92Eg/xcexaT)+Vs/xcexexp(qVs/xcexaT)
xcex1=Electrical conductivity at room temperature when no voltage is applied to the MIM nonlinear device;
xcex2=Sharpness of the voltage-current characteristic;
Vi=Voltage applied to the insulating film;
Eg=Activation energy;
xcexa=Bolzmann""s constant;
T=Absolute temperature;
Vs =Voltage applied to the interface between the first layer and the second layer of the insulating film;
xcex=Constant;
q=Electric charge of an electron;
In the above formula, the first term relates to the conduction of the insulating film according to the Pool-Frenkel conduction, and the second term relates to energy difference between the conductive segments in the insulating film. That is, the second term is based on regular-directional conduction to be a pn junction when the first layer is assumed to be an n-type semiconductor and the second layer is assumed to be a p-type semiconductor.
As described above, the MIM nonlinear device of the present invention has advantages such that the nonlinear coefficient (xcex2 value) is large, sharpness of the voltage-current characteristic is excellent, change of the voltage-current characteristic is small, and the reliability is high.
Furthermore, the liquid crystal display panel of the present invention includes the above-mentioned MIM nonlinear device and, more specifically, includes a transparent substrate, a signal line disposed on the substrate in accordance with a predetermined pattern, a plurality of MIM nonlinear devices of the present invention connected to the signal line, a first substrate with pixel electrodes connected to the MIM nonlinear devices, a second substrate having another signal line in a position facing the pixel electrodes, and a liquid crystal layer sealed between the first substrate and the second substrate.
According to this liquid crystal display panel, images can be displayed at a high contrast without sticking, and therefore with high quality. The liquid crystal display panel can, therefore, be used for a large variety of purposes.